Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.2.3.1 Hydrological Processes Related to Ice and Snow ====

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Declining ice-sheet mass, glacier extent and Northern Hemisphere (NH) sea ice, snow cover and permafrost ( [[#Collins--2013|Collins et al., 2013]] ; [[#Vaughan--2013|Vaughan et al., 2013]] ) is an expected consequence of a warming climate (Sections 2.3.2, 3.4, 4.3.2.1 and 9.3 – 9.5). A decline in mountain snow cover and increased snow and glacier melt will alter the amount and timing of seasonal runoff in mountain regions (Sections 3.4.2, 3.4.3 and 9.5). Earlier and more extensive winter and spring snowmelt (X. [[#Zeng--2018|Zeng et al., 2018]] ) can reduce summer and autumn runoff in snow-dominated river basins of mid–high latitudes of the NH (Rhoades et al., 2018; [[#Blöschl--2019|Blöschl et al., 2019]] ). Since AR5, an earlier but less rapid snowmelt has been explained by reduced winter snowfall and less intense solar radiation earlier in the season (Musselman et al. , 2017; Wu et al. , 2018; Grogan et al. , 2020). Reduced snow cover also increases energy available for evaporation, which can dominate declining river discharge based on modelling of the Colorado River ( [[#Milly--2020|Milly and Dunne, 2020]] ). An increase in the fraction of precipitation falling as rain compared with snow can lead to declines in both streamflow and groundwater storage in regions where snowmelt is the primary source of recharge ( [[#Earman--2011|Earman and Dettinger, 2011]] ; [[#Berghuijs--2014|Berghuijs et al., 2014]] ). Such regions include western South America and western North America, semi-arid regions which rely on snowmelt from high mountain chains ( [[#Ragettli--2016|Ragettli et al., 2016]] ; [[#Milly--2020|Milly and Dunne, 2020]] ). Rain-on-snow melt events reduce at lower altitudes due to declining snow cover but increase at higher altitudes where snow tends to be replaced by rain based on observations and modelling (Musselman et al., 2018; [[#Pall--2019|Pall et al., 2019]] ), thereby altering seasonal and regional characteristics of flooding ( [[IPCC:Wg1:Chapter:Chapter-11#11.5|Section 11.5]] ).

Seasonal melt water from high mountain glaciers in Asia (see Cross-Chapter Box 10.4) supply the basic needs of 221 ± 97 million people (Pritchard, 2019; [[#Immerzeel--2020|Immerzeel et al., 2020]] ). Glacier-melt in response to warming can initially lead to increased runoff volumes, especially in peak summer flows, but they will eventually decline as most glaciers continue to shrink. SROCC concluded there is ''high confidence'' that the peak runoff has already been passed for some smaller glaciers ( [[#Hock--2019a|Hock et al., 2019a]] ). Increased precipitation and glacier-melt can also contribute to rising lake levels and flood hazards in regions such as the inner Tibetan Plateau, Patagonia, Peru, Alaska and Greenland (Lei et al. , 2017; Shugar et al. , 2020; Stuart-Smith et al. , 2020) . Since AR5, evidence from multiple locations (New Zealand, Greenland, Antarctica) shows that intrusions of warm, moist air are important in controlling glacier mass balance, the likelihood of extreme ablation or snowfall events depending on air temperature (Gorodetskaya et al. , 2014; Mackintosh et al. , 2017; Mattingly et al. , 2018; Little et al. , 2019; Oltmanns et al. , 2019; Wille et al. , 2019; Adusumilli et al. , 2021) . Sensible heating from warm air and increased longwave radiation from atmospheric moisture and low clouds drive melt events (Stuecker et al., 2018).

Reductions in snow, freshwater ice and permafrost affect terrestrial hydrology. Permafrost degradation reduces soil ice and alters the extent of thermokarst lake coverage ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ; M. [[#Meredith--2019|]] [[#Meredith--2019|Meredith et al., 2019]] ). A lag between current climate change and permafrost degradation is expected, given the slow response rates in frozen ground and the fact that snow cover insulates soil from sensible heat exchanges with the air above (Hoegh-Guldberg et al. , 2018; García-García et al. , 2019; Soong et al. , 2020) . Post‐wildfire areas are also linked with permafrost degradation in the Arctic based on satellite observations ( [[#Yanagiya--2020|Yanagiya and Furuya, 2020]] ). An increase in spring rainfall can increase heat advection by infiltration, exacerbating permafrost thaw and leading to increased methane emissions ( [[IPCC:Wg1:Chapter:Chapter-5#5.4.7|Section 5.4.7]] ; [[#Neumann--2019|Neumann et al., 2019]] ). Increased heat transport by Arctic rivers can also contribute to earlier sea ice melt ( [[#Park--2020|Park et al., 2020]] ).

In summary, it is ''virtually certain'' that warming will cause a loss of frozen water stores, except in areas where temperatures remain below 0°C for most of the year. There is ''high confidence'' that warming and reduced snow volume drives an earlier snowmelt, leading to seasonally dependent changes in streamflow. There is ''medium confidence'' that weaker sunlight earlier in the season can reduce the rate of snowmelt. Melting of snowpack or glaciers can increase streamflow in high-latitude and high-altitude catchments until frozen water reserves are depleted ( ''high confidence'' ). There is ''high confidence'' that warm, moist airflows and associated precipitation dominate glacier mass balance in some regions (New Zealand, Greenland, Antarctica).

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